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Ranking geo-engineering options for mitigating climate change impacts

Fig 1. Comparisons of aspects of five geo-engineering proposals
Fig 1. Comparisons of aspects of five geo-engineering proposals

I recently came upon this interesting mini-review in Nature Geoscience which looked at the cost-effectiveness of different geo-engineering options for mitigating climate change impacts (for an earlier discussion on BNC, see here). The paper is entitled “Ranking geo-engineering schemes“, by New Zealander Philip Boyd. A full-text PDF version of the article is available here for download. Here is the abstract, and a few snippets:


Geo-engineering proposals for mitigating climate change continue to proliferate without being tested. It is time to select and assess the most promising ideas according to efficacy, cost, all aspects of risk and, importantly, their rate of mitigation. …        Appraising the relative merits of geo-engineering designs for a purposeful perturbation of the Earth system is essential: funds to investigate such proposals in detail are limited, and not all schemes can be put in place if we are to monitor the Earth system’s response to each scheme with any confidence. …        This possibility of unwanted side-effects must be factored into the cost of schemes (Fig. 1). In addition, unintended changes in the Earth system could, to an unknown degree, cancel out the mitigation of climate change driven by geo-engineering, causing a reduction in the estimated efficacy of a scheme and an increase in its cost. …        Up to now, the relative merits of various geo-engineering schemes have mainly been discussed in the context of risk and cost, with a few reports on individual schemes also looking at efficacy. But restricting an evaluation to these three factors is of limited value. Two disparate recent studies, one using climate modelling to explore the implications of delaying climate mitigation, the other on designing a global response plan to confront climate change, suggest that relief from climate warming will be needed very soon. The timescale to advance each scheme from development to implementation to verification and hence mitigation is therefore of primary importance. If geo-engineering is to have a role in stabilizing our climate, we must apply metrics that incorporate efficacy, cost, risk and time in order to rank where future research effort is best focused. …        Funding research into only a few promising schemes, according to such metrics, may lead to one or two relatively reliable mitigation options that can be placed in a ‘climate-change toolbox’. In the near future, we must decide the relative importance of time, cost, risk and efficacy in tackling climate change if it is decided to press ahead with a geo-engineering approach. Of course, it could transpire after such an analysis that climate mitigation strategies with a very low risk but apparently higher costs, such as direct carbon capture and storage, are the best approach. As the costs of inaction and of delaying the mitigation of climate change are rising, an initial high investment — matched with a very low risk — may seem more and more reasonable.


Options considered by Boyd in his trade-off analysis include carbon burial (long-term physical storage of atmospheric CO2, under pressure, below the Earth or within the deep ocean), geochemical carbon capture (dissolving CO2 in bicarbonate ions in seawater or in solid form such as limestone), atmospheric carbon capture (wind scrubbers using chemical absorbents – artificial trees), ocean fertilisation (enriching surface waters with iron or other nutrients to promote phytoplankton growth, with the hope that the extra carbon captured via photosynthesis would then mix with the deep ocean), stratospheric aerosols (injection of sulphur particles into the stratosphere to reflect incoming sunlight to space, simulating the volcano effect), cloud whitening (spraying seawater droplets skywards to simulate the ship contrail effect), and sunshades in space (rocketing off a huge number of mirrors into space to intercept sunlight at the Lagrange point [see below for discussion of the probable impact of this]).

Mongabay has also written an overview here, and The Oyster’s Garter here.

Overall, I judge the paper to represent an interesting and logical thought experiment — acknowledging that the situation with now face, with so much warming in the pipeline, is already sufficient bad to require hard-nosed evaluation of planetary scale ‘triage’ to reduce the ‘fever’. Otherwise, we may never get back important features of the Earth system such as Arctic sea ice, nor stave off the extinction of many species already under stress from human impacts for which climate change becomes the straw that breaks the camel’s back.

Needless to say, no one would credibly argue that geo-engineering is a replacement for mitigation of carbon emissions. A business-as-usual scenario of coal burning, taking atmospheric CO2 to 750 to >1000 ppm (directly or via carbon-cycle feedbacks), will force the climate system so far out of whack that no ‘patch up job’ will be sufficient. No, the context under which geo-engineering might need to be considered is if a measured analysis shows that even with major emissions reductions, the  impacts of committed warming will be so bad as to warrant using additional ‘terraforming’ of planet Earth. Are we at that point already? Dunno. But let’s have that risk assessment and necessary R&D done, just in case.

Finally, a biologically related question. Can ‘geo-engineering’ protect ecosystems and humanity from climate change impacts, should global warming start to run out of control?

Well, not really, at least according to another paper by Lunt and co-authors (get the pre-print full text here) entitled ” ‘Sunshade World’: A fully coupled GCM evaluation of the climatic impacts of geoengineering“. In a fascinating application of a Global Climate Model (HadCM3L), these authors take a hard look at the impacts of the sunshades-in-space idea described briefly above. That is, the (expensive and logistically challenging) option of installing a few trillion 1m diameter reflective mirrors between the Earth and the Sun, to reduce incoming solar radiation by 2-5%. Costs and logistics aside, would this mitigate climate change impacts?

The answer is complex, but the upshot is that such a geo-engineering solution-of-last-resort would seem to create as many problems as it solves. The tropics would cool, which might spare rain forest biomes or cause them to revert to savanna, but polar amplification of the warming is predicted to continue, leading to the elimination of Arctic sea ice and the probable continued destabilisation of land-based polar ice sheets.

This solution could avoid major heat waves that threaten coral reef systems with bleaching. The global hydrological cycle would likely become less intense, with the atmosphere being drier overall. However, ocean acidification due to high CO2 would be unaffected by this geo-engineering, and this impact alone is likely to be catastrophic for species such as corals, forams and pteropods that secrete a calcite or aragonite skeleton, potentially disrupting entire strands of the marine food web.

Interestingly, the authors speculate that the sort of conditions implied by this scenario (lower total solar irradiance and high atmospheric CO2 concentration) would have the side effect of re-creating a world similar to the Cambrian period, 500 million years ago – the dawn of the Phanerozoic, when visible life first became abundant.

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By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

15 replies on “Ranking geo-engineering options for mitigating climate change impacts”

I distinguish SACTCAR solutions from BTRO ones. If good BTRO options exist, and they do, painstaking investigation of SACTCAR ones is idle. Guessing those acronyms is left as an exercise.

Mirrors in space are obviously SACTCAR, like sulphate aerosols. Interesting fact: one BTRO method has been demonstrated on a considerable scale inadvertently. This is probably 99.9 percent of the CCS that has actually happened.

Doing it on a gigatonne scale, on purpose, is one of the geochemical schemes that Boyd apparently considers to be hard to stop on short notice. What if it someday turns out that the project of easing the atmospheric CO2 concentration back to the preindustrial level must be brought to an emergency stop? What if, in 2025, having with much effort got the atmospheric CO2 concentration back down to 370 ppm despite China’s having put up 150 gigatonnes of CO2 in the 20-teens, we then find that the CO2 level must be allowed to start rapidly rising again, like now?

Seems like a very remote possibility, and anyway, what is the difficulty with turning off a lot of rock crushers and blowers?

— G.R.L. Cowan (How fire can be domesticated)


Barry, on a related topic of emissions pathways that contemplate technologies for atmospheric carbon capture allowing for a carbon negative future, Bill Hare has a really interesting chapter in the State of the World 2009, available at

He suggests getting fossil CO2 emissions down to close to zero in 2050 and being carbon negative thereafter – a commitment to action that spans centuries – to return global temperatures to beneath 1 degree Celsius warming “is plausible technically” and “goes beyond the technically and economically feasible pathways published elsewhere”.


Enhanced weathering is a geochemical scheme which is highly controllable. The costs are such that somewhat less than 1% of the gross world product, about equal to the sum of world military budgets, would suffice to slightly more than remove all of the current (2007 CE) excess carbon dioxide added by humans.

Well, maybe slightly more. Still working out all the costs.


G.R.L. Cowan – what’s your argument or scenario? I think of us as hitting peak CO2 in the mid-2020s, after new global temperature records in the early 2010s lead people to consciously aim at a global economy of zero net emissions, and not just reduced emissions.


See albedo discussion.

Some geoengineering wants to reduce CO2, a Good Thing.
Others want to increase albedo, to reduce heat, at least in part to lessen the bad positive feedbacks.

For the latter, I’d observe that synthetic volcanoes or mirrors in orbit reduce sunlight over large amounts of the world, *including g*:

– agricultural land
– solar PV and solar thermal

I don’t mind the ships that do clouds in the ocean, but I’d sure rather let sunlight come at 100% in many places, and *use* more of it [for electricity or heating water or growing things], or at worst, cover unusable land with mirrors. The post has some numbers about albedo requirements.

Put another way, I think the {sulfates, satellite} albedo-increase path is dominated by potential terrestrial solutions. They don’t reduce the CO2, but maybe they could hold off tundra melt.


I’ve been doubtful of space based geoengineering proposals and suspect many proponents want massive space capability as their primary aim and are looking for good sounding reasons to get the world at large to fund it. Some may see civilisation on Earth as ultimately doomed and getting lots of people into space is about species survival – although I’ve never been convinced that humanity would easily thrive in the absence of the complex, mostly natural ecosystem that is our home planet and without the complex economy of modern Earth based civilisation. Sustaining the high tech economy needed to survive independently with less than tens of millions of people and without a massive economy nearby for finance, technology and a market for the supposedly easily accessed mineral resources seems farfetched. It leaves me tending to dismiss the space mirror proposals out of hand, perhaps unfairly, just for distrusting the motivations of it’s proponents. Apart from that, the sound bite versions seems to gloss over the requirement for each mirror to be powered to keep it in place and to track accurately – they can’t be put in place and left to their own devices and will need repair and maintenance. Like space based solar power satellites, the cost is likely to be prohibative. Even so I think it’s good to explore some visionary ideas. Some of that stuff is going to be needed, for example I admit to ongoing curiosity as to the suitability of the power transmission aspects of space solar as a backbone for a global grid. Then, I also wonder if HVDC or superconductors could successfully cross oceans as well as continents – SC cables suspended from floating platforms that house the required refrigeration? Vanadium redox undersea pipelines? Even higher voltage HVDC with even greater distances and even lower transmisson losses?

But back to geoengineering, the Earth and ocean based version – I expect it will be needed, mostly because of the heels dug in unwillingness of humanity to face AGW head on, even that proportion of humanity that takes the issue seriously. By the time it is treated with the urgent seriousness it deserves, atmospheric levels of GHG’s will be much more than present and be a serious danger; the need to get them out of the atmosphere when major sinks are saturated or turning into sources as warming causes their existing load to be released.


David Benson said,

Enhanced weathering is a geochemical scheme which is highly controllable. The costs are such that somewhat less than 1% of the gross world product, about equal to the sum of world military budgets, would suffice to slightly more than remove all of the current (2007 CE) excess carbon dioxide added by humans.

Well, maybe slightly more. Still working out all the costs.

Did you get answers to your questions at Rabett Run? I looked at investing 50 kJ/(mol CO2) to do two things: comminute olivine to 25 microns (40 kJ) and lift the powder 5 km above the grinding site (10 kJ), so that as they settle at 0.06 m/s the wind can carry them far.

If their magnesium becomes magnesium bicarbonate dissolved in the sea, they take down twice the CO2 my calculations were based on.

I see no reason why CO2 wouldn’t eat them right to the centre of a grain. At the weathering rate is given. Ferrous iron, as it gets oxidized to ferric, helps break up a grain, even though I don’t think it binds any CO2.

— G.R.L. Cowan (How fire can be domesticated)


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